1. Field of the Invention
The present invention relates to electrophotographic image forming apparatuses such as copiers, printers, facsimile machines, and multifunction peripherals having functions of the aforementioned devices.
2. Description of the Related Art
In fixing devices that use heat and pressure, a pressure member is pressed against a fixing member having a heat source provided inside. A transfer sheet with an unfixed toner image is passed through the part between the fixing member and the pressure member, to fix the toner onto the transfer sheet (a recording medium).
The surface of the fixing member is covered by, for example, fluorine coating, to prevent toner from adhering to the fixing member. However, depending on various conditions relevant to the environment or the recording medium, a slight amount of toner on the recording medium may adhere to the fixing member, which is referred to as offset. The offset toner remains on the fixing member and the pressure member that contacts the fixing member. This toner may transfer to the recording medium and soil the image on the recording medium. To prevent such a situation, the fixing member is provided with a cleaning member such as a cleaning web or a cleaning roller.
The cleaning member is pressed against the fixing member and the pressure member to collect the offset toner. To collect the offset toner, particularly, in a reliable manner, a cleaning web involving a roll-up operation is used for appropriately collecting the toner, with which high cleaning performance can be constantly achieved.
A cleaning web is typically impregnated with a small amount of silicone oil (for example, 5 g/m2 through 10 g/m2) for the purpose of achieving high cleaning performance. However, when the oil is unevenly applied, the resultant image may have uneven gloss.
Furthermore, when the cleaning web is brought in contact with the pressure roller, the oil that is unevenly applied on the pressure roller may be transferred to the fixing belt or the fixing roller. In this case, because the cleaning web is disposed at the pressure roller, the fixing belt or fixing roller does not contact the cleaning web. Therefore, the unevenness on the fixing belt or fixing roller cannot be evened out by contacting the cleaning web immediately after performing a fixing operation of one sheet. Accordingly, the resultant image may have a significant level of uneven gloss. A supplying roller stores the cleaning web in a rolled-up state. The amount of impregnated silicone oil increases toward the center of the diameter of the rolled-up cleaning web. That is to say, the amount of impregnated silicone oil increases toward the end of the operating life of the cleaning web.
To address this problem, patent documents 1 and 2 disclose mechanisms for separating the web contact roller from the contact object while the machine is in a standby state, so that excessive oil is prevented from exuding during the standby state. However, a link mechanism or an actuator mechanism is required for the separating motion, which requires high cost. Furthermore, when there is excessive silicone oil while the machine is operating, effects of this mechanism cannot be expected to be achieved. Patent document 3 discloses a method of providing a cleaning web including silicone oil and a cleaning web that does not include silicone oil. The cleaning webs are brought into contact or separated from the object according to fixing conditions, so that unevenness in gloss caused by silicone oil is eliminated. However, this method requires complex control operations, and requires high cost, as apparent from the configuration. Furthermore, in cases where the cleaning web including silicone oil needs to be used for achieving high cleaning performance, effects of this method cannot be expected to be achieved.
Among the above-described fixing devices, some devices have a function of moving the pressure member with respect to the fixing member for applying pressure, reducing pressure, and not applying pressure (by separating the pressure member from the fixing member). This function is used for the purpose of preventing the degradation of the fixing member and the pressure member. Specifically, the members are separated when the device is not used so that load is not applied to the members. Furthermore, the members may be separated for the purpose of cooling only the pressure member. Furthermore, some devices have a function of implementing plural positional relationships between the pressure member and the fixing member when the pressure member is pressed against the fixing member, such as strong press-contact and weak press-contact. This function is used for the purpose of achieving the optimum press-contact state according to the environment and conditions of the recording medium.
In a configuration where the cleaning member (contact roller) is brought into contact with a movable pressure member as described above, when the length between the contact roller and the pressure member increases and the cleaning web is not sufficiently pressed against the pressure member, the cleaning function cannot be sufficiently implemented and consequently the image is soiled. Furthermore, when the length between the contact roller and the pressure member decreases and the cleaning web is excessively pressed against the pressure member, the materials forming the contact roller and the pressure member are degraded, and the torque required for rolling up the cleaning web increases, and the cleaning web may not be properly retrieved.
That is to say, when the positional change of the pressure member is sufficiently small at the location where the cleaning member contacts the pressure member, the cleaning member can be completely fixed with respect to the fixing device. However, when the pressure member is configured to separate from such a location, the positional change of the pressure member is large. Thus, the toner cleaned off from the pressure member may fly out, transfer to the recording medium, and cause failures.
Furthermore, there may be cases where a large amount of toner enters the cleaning nip part where the cleaning web is pressed against a member that is the object of cleaning (cleaning object), due to factors such as a fixing failure occurring while the fixing device is operating or a paper jam occurring at the fixing nip. In such a case, a large amount of melted or half-melted toner is expected to accumulate at the cleaning nip. When a large amount of melted or half-melted toner is accumulated at the cleaning nip, and the fixing device stops operating and is left to cool down until the toner is fixed on the cleaning web and the cleaning object, the following problem may arise. That is, when the fixing device starts operating once again, the cleaning web may be unexpectedly discharged from the roll-up roller (that rolls up the used web to retrieve the used web) and the supplying roller (that supplies a new web).
When the web is unexpectedly discharged, the web may sag and tangle with surrounding components, thus causing various operational failures. For example, when the cleaning object is a pressure roller, the web may be wound around the pressure roller. In another example, the web may tangle with a separation mechanism for separating the recording medium with fixed toner from the fixing member. As a result, the separation mechanism may not be able to separate the recording medium from the fixing member. Furthermore, when the web is rolled out from the roll-up roller and the web sags, a web that is soiled from being used for a cleaning operation may be used once again for cleaning. Accordingly, a cleaning failure is caused, which leads to abnormal images.
To prevent the web from being discharged as described above, there are means for preventing reversal, such as providing the roll-up roller with a one-way clutch mechanism, or using a worm gear as the roll-up roller driving system. However, as the roll-up roller retrieves the used web, the diameter of the rolled-up cleaning web increases. Therefore, the torque required for retrieving the web continuously increases assuming that the web is pulled with a constant force at the cleaning nip part. Under such circumstances, when a large amount of unfixed toner enters the cleaning nip as described above, and conditions by which toner is fixed are fulfilled, excessive load is disadvantageously applied on the means for preventing reversal. Furthermore, even if the means for preventing reversal can withstand the torque, if the used web is loosely rolled-up by the roll-up roller, the web that is loosely rolled-up may be discharged in the direction opposite to the web roll-up direction under the conditions in which toner is fixed. Consequently, the used web may enter the cleaning nip part once again.
A method of adding some means for preventing reverse rotation to the shaft of the roll-up roller (see, for example, patent document 4) is typically known as a means for preventing reversal. This method is for adding a reversal torque to the roll-up roller shaft, and therefore as the diameter of the roll-up roller increases, the required torque increases.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-215838
Patent Document 2: Japanese Laid-Open Patent Publication No. H08-152801
Patent Document 3: Japanese Laid-Open Patent Publication No. 2009-025519
Patent Document 4: Japanese Laid-Open Patent Publication No. 2008-01544